This invention relates to heat engine control systems, and in particular to a control system for a Cheng dual-fluid engine.
U.S. Pat. No. 4,128,994 and pending U.S. patent application, Ser. No. 967,108, Regenerative Parallel Compound Dual-Fluid Heat Engine, (referred hereinafter collectively as the prior Cheng cycle patent) describe the dual-fluid (Cheng) cycle heat engine. This engine, which employs parallel Rankine and Brayton cycles, requires a critical balance of operating parameters to produce high thermal efficiencies. For any given set of cycle parameters, the prior Cheng cycle patent referred to above, teaches that an efficiency peak exists only at a unique ratio of Rankine to Brayton fluids. Either too much or too little Rankine fluid leads to reduced cycle efficiency.
The prior Cheng cycle patent defines the peak operating condition cycle parameters to design an engine for 100 percent load. Because of the parallel combined nature of the Brayton and Rankine cycles in this engine, the quantity and quality of steam that can be generated by a given engine configuration can be varied freely over a range. The control path for the steam cycle is essentially independent of that for the gas turbine cycle. The control path for throttling the engine is essentially free or undefined. Thus to reduce engine power from the peak operating points to reach partial load output conditions poses a difficult control problem that involves precision control of the air flow, fuel flow, and steam flow.
In addition, because of the nature of the parallel compound fluid engine, several independent parameters are defined somewhat arbitrarily by the designer or fixed by some operational constraint such as synchronous speed of a generator for example. These include the compression ratio (CPR), turbine inlet temperature (TIT), compressor RPM and work turbine RPM, as well as those determined by the air, fuel and steam flows, which are air-fuel ratio (A/F), specific heat input rate (SHIR), steam-to-air ratio (X.sub.mix), and total mass flow. Among the constraints on operating this engine at variable load conditions are the boiler surface area, boiler pressures, and the degree of superheat of the steam. Taken together this array of parameters makes design of a control system both difficult and unique.
The waste heat boiler for the dual-fluid engine system is normally designed for the peak efficiency condition at design load. Of course, once the heat exchanger is built, the surface area for the heat exchanger is fixed. If one desires to operate the engine at over-load conditions, the required surface area to generate more steam is not available unless the system has been designed with a boiler that is oversized for the design load condition. On the other hand, when the engine is operated under partial load conditions, the area of the heat exchanger is in excess of needs, thus permitting operation at decreased differences in exhaust gas and boiler temperature.
For a given turbine inlet temperature and compression ratio, peak work output efficiency of the dual-fluid cycle engine occurs only at a certain steam-to-air ratio. That ratio of steam-to-air is precisely defined as corresponding to maximum recovery of exhaust heat by the steam within designated turbine temperature limits of the engine. Steam is generated by recovering the exhaust waste heat at pressures that are relatively low when compared to the pressures usually used in a steam Rankine cycle following a gas turbine, the so-called combined gas/steam (COGAS) system.
In the Cheng dual-fluid cycle system the steam is injected into the engine before the work turbine and both combustion gases and steam deliver work to the turbine. Since the energy of the steam is derived from the exhaust of the same work turbine, or turbines, the system contains a feedback loop which must be solved in designing a control system.
The Cheng cycle is complicated in other ways. Unlike a gas turbine engine the exhaust temperature of the Cheng cycle turbine at a given inlet temperature and fixed pressure ratio is no longer uniquely defined by the turbine characteristics. It also depends on the steam-air mixture, X.sub.mix. Steam and combustion air have different thermodynamic properties, namely, specific heats, and their ratio. Air has a higher gamma function, i.e., specific heat ratio, than steam. In expanding a mixture of combustion air and steam through a turbine more work is produced for a given pressure ratio expansion than can be produced by expanding the air and the steam separately through the same pressure ratio.
The details of this synergistic effect were disclosed in the prior Cheng patent. As discussed there the peak efficiency can be identified with a minimum "effective" temperature. But because the "effective" temperature is a measure of the thermodynamic potential that cannot be directly measured by a thermometer or thermocouple device, the feedback control design is even more difficult. In this invention a control system is disclosed to resolve these difficulties.
As disclosed in the prior Cheng patent, the maximum heat recovery rate does not occur at the lowest waste heat boiler gas exit temperatures. The latent heat of evaporation of the steam in the mixture gas is generally not recovered. Physically, if too much steam is used, the exit (engine injection) temperature of the steam from the waste heat boiler is low due to the large amount of water used to recover the waste heat.The heat loss due to the latent heat content of the exhaust gas exiting the boiler is very large. On the other hand, if the steam quantities are insufficient the heat exchanger exit temperatures of the exhaust products become excessive, and the engine will not have reached its improved efficiency potential. For a given set of parametric constraints the peak efficiency occurs at the steam-air ratio corresponding to the maximum rate of waste heat recovery. This is not known unless the constraints on the boiler design are given.
Traditionally, prior art control systems for gas turbines adjust for the load on the gas turbine by merely varying the injection rate of fuel, thereby increasing or decreasing the turbine inlet temperature. According to thermodynamic laws a higher working temperature provides not only higher work content but, generally, a higher thermal efficiency. One would presume that in the dual-fluid cycle however, the maximum continuous-operation turbine inlet temperature corresponds to the maximum efficiency design point of the engine. One would also presume that the partial load condition could be obtained by merely reducing the amount of fuel and steam injected into the engine system while maintaining the maximum turbine inlet temperature. However, neither of these presumptions are correct.